Optical communication systems are a substantial and fast-growing constituent of communication networks. The expression "optical communication system," as used herein, relates to any system which uses optical signals to convey information across an optical waveguiding medium. Such optical systems include, but are not limited to, telecommunications systems, cable television systems, and local area networks (LANs). Optical systems are described in Gowar, Ed. Optical Communication Systems, (Prentice Hall, NY) c. 1993, the disclosure of which is incorporated herein by reference. Currently, the majority of optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides. To convey information from plural sources, time-division multiplexing is frequently employed (TDM). In time-division multiplexing, a particular time slot is assigned to each information source, the complete signal being constructed from the signal portion collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, its capacity is limited by fiber dispersion and the need to generate high peak power pulses.
While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded, e.g., by laying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. A WDM system employs plural optical signal channels, each channel being assigned a particular channel wavelength. In a WDM system, optical signal channels are generated, multiplexed to form an optical signal comprised of the individual optical signal channels, transmitted over a single waveguide, and demultiplexed such that each channel wavelength is individually routed to a designated receiver. Through the use of optical amplifiers, such as doped fiber amplifiers, plural optical channels are directly amplified simultaneously, facilitating the use of WDM systems in long-distance optical systems.
In many applications, such as optical LANs, cable television subscriber systems, and telecommunications networks, there is a need to route one or more channels of a multiplexed optical signal to different destinations. Such routing occurs when optical channels are sent to or withdrawn from an optical transmission line e.g., for sending optical channels between a terminal and an optical bus or routing long distance telecommunications traffic to individual cities. This form of optical routing is generally referred to as "add-drop multiplexing."
One approach to add-drop multiplexing is explored in Giles and Mizrahi, "Low-Loss ADD/DROP Multiplexers for WDM Lightwave Networks," IOOC Technical Digest, (The Chinese University Press, Hong Kong) c. 1996, pp. 65-67, the disclosure of which is incorporated herein by reference. In this paper, an add-drop multiplexer is proposed which uses two three-port optical circulators with a fiber grating positioned between the two optical circulators. Using this configuration, an optical signal to be dropped from an optical transmission path is reflected by the narrowband fiber grating and exits through the drop port of the optical circulator. All other input signals exit via the through port of the optical circulator. Similarly, an optical signal to be added which has a wavelength nominally identical to the optical signal being dropped from the optical transmission path is input to the add port of the second circulator. The signal to be added to the optical transmission path is reflected towards the though port of the second circulator by the same fiber grating used for signal dropping.
Although the disclosed add-drop multiplexer is adequate for WDM optical systems with greater than about 100 GHz channel spacings (approximately 0.8 nm at 1550 nm wavelength), problems can arise in WDM systems having a closer channel spacing, such as 50 GHz. This close channel spacing may be required for large channel count systems or for long-haul systems. Consequently, any grating used to select a channel must be sufficiently narrow to avoid overlapping the spectral territory of an adjacent channel. FIG. 1 depicts the transmission spectra of a strong grating. Because strong gratings, i.e., those gratings which reflect over approximately 99% of the incident design wavelength, reflect light over a region greater than about 0.8 nanometer (as seen in FIG. 1), they can interfere with the light from adjacent channels. However, when weaker, narrower gratings are used, an example of which is depicted in FIG. 2, a significant portion of the incident signal will be transmitted. If such a grating is used in an add-drop multiplexer, a portion of the incident channel to be dropped will pass through, resulting in coherent crosstalk with the channel of the same wavelength which is added by the multiplexer. Generally, to limit this type of coherent crosstalk it is desirable for attenuation of the dropped optical channel to be greater than about 30 dB, particularly greater than about 40 dB.
Consequently there is a need in the art for add-drop multiplexers which are compatible with dense WDM optical communications systems, particularly those systems which have greater than about 30 optical channels.